The prior art is illustrated by United States patents U.S. Pat. No. 3,201,491, International patent WO 96/12542, European patent EP-A-0 787 517 and U.S. Pat. No. 3,268,605.
A simulated counter-current or simulated moving bed adsorption separation process must include:
an assembly of beds of adsorbent (or fixed phase) disposed in a closed loop, the total number of beds generally being in the range three to twenty-four. PA1 Means for connecting each of these beds with streams feeding the separation unit, such as solvent(s), feed(s) to be separated, or an internal reflux. PA1 Means for connecting each of the beds with streams issuing from the separation unit, such as extract(s), raffinate(s). PA1 Means for connecting each of the beds with the following bed (in the direction of fluid circulation). PA1 Each bed is contained in an independent receptacle and a line connects it to the following bed. In such a case, it is advantageous to connect an independent line per inlet and outlet stream to that line. The AROMAX process from TORAY Industries falls into this process category and constitutes a large scale embodiment. While the individual beds are disposed in a single column, they are separated by solid plates, and all of the fluid from each bed is collected by a line which leaves the column and then returns to feed the following bed. More generally for small units, it is convenient to isolate each bed in a particular receptacle and to connect it to the following bed by a line. In that type of embodiment, it is easier to use one line per inlet or outlet stream and per bed, thus avoiding any problems of contamination of one stream be another. PA1 Several beds are contained in a single receptacle or column and each bed is separated from the following bed by a distributor plate which must carry out four functions:
Depending on the particular embodiment, either a fluid recirculation unit is placed between the last and first bed (pump or compressor), or an apparatus which can interrupt the stream is placed between two successive beds (on-off valve), or an apparatus which can regulate the flow rate and/or pressure is located at the inlet to each bed.
Two types of embodiments are available:
collect the fluid which gathers at the end of the bed; PA2 withdraw a portion of the fluid during certain phases of the cycle so that the composition of the withdrawn portion is identical to the composition of the fluid as a whole; PA2 inject an external fluid into the internal fluid during certain phases of the cycle and form a mixture which is homogeneous in composition; PA2 distribute the fluid to the inlet to the following bed.
The PAREX process described in U.S. Pat. No. 2,985,589 shows how it is possible to connect at least two inlet streams and two outlet streams sequentially, by means of a valve, to each of the beds containing adsorbent.
That patent clearly discloses each distributor connected via a single line to a valve which successively connects each distributor to the feed then to the extract, then to the solvent, then to the raffinate.
Such a procedure has the disadvantage of considerably reducing the performance of the process (purity and yield) since each stream is contaminated by the contents of the common line when a given stream is connected to a particular bed.
Purely by way of illustration, let us consider a column sub-divided into twelve beds and operating as follows:
In period n.degree.1 of the cycle (which comprises 12 periods), solvent is injected into bed n.degree.1, extract is withdrawn from bed n.degree.3, feed is injected into bed n.degree.7 and raffinate is withdrawn from bed n.degree.9. At the start of the second period of the cycle, the injection and withdrawal points are displaced by one bed and solvent is thus injected into bed n.degree.2, extract is withdrawn from bed n.degree.4, feed is injected into bed n.degree.8 and raffinate is withdrawn from bed n.degree.10. The cycle continues until the twelfth period where solvent is injected into bed n.degree.12, extract is withdrawn from bed n.degree.2, feed is injected into bed n.degree.6 and raffinate is withdrawn from bed n.degree.8.
At the end of the first period, the line connecting distributor n.degree.1 to the valve managing the streams is thus full of solvent. At the start of the fourth period, raffmate is withdrawn from bed n.degree.12, via the line through which solvent was injected during period 1. During the time required for evacuating the volume of that line, solvent is removed instead of raffinate. This results in needless dilution which results in over-consumption of solvent and thus increased operating costs. At the end of the fourth period, that line is full of raffinate. During the seventh period, feed is injected into bed n.degree.1. For a certain time, raffinate is forced back into bed n.degree.1 instead of injecting feed. This results in the para-xylene in the feed being diluted by raffinate. Since the productivity of the unit is proportional to the concentration of para-xylene in the feed, the overall result is a drop in productivity. At the end of the seventh period, the line is full of feed. During the tenth period, extract is withdrawn from bed n.degree.12. During the time required to flush the line, feed is extracted instead of extract. This results in a very large reduction in purity since the para-xylene content in the feed is of the order of 20% (thus 80% impurities). Such contamination renders the production of very high purity paraxylene impossible. At the end of the tenth period, the line is full of extract. At the start of the first period, solvent forces the contents of the line back into the adsorber at the start of the para-xylene desorption zone (zone 1). A portion of the para-xylene thus remains adsorbed on the fixed phase, and is subsequently partially eluted in the raffinate stream. This results in a loss of para-xylene yield.
In summary, contamination due to the use of a common line connecting each of the four streams of the process to the inlet to the distributor plate causes and increase in solvent consumption, a reduction in productivity, a very large reduction in purity and a reduction in yield.
While the PAREX process provides an injection step between withdrawing extract and injecting feed, of the only two documents published on this subject, one concerns injecting one or two internal reflux streams of extract or distilled raffmate located either between the extract and the feed, or between the feed and the raffmate (U.S. Pat. No. 3,761,533) and the other concerns injecting a flushing fluid of indeterminate nature at a position which is also indeterminate (U.S. Pat. No. 4,434,051). Re-injecting extract or solvent between the feed and the extract only solves the problem of a large reduction in purity.
In our ELUXYL process, instead of using a single valve to connect the different streams and each bed, one on-off valve per stream and per bed is provided.
While each of the valves is located as close as possible to the bed it serves, each distributor still has to be connected to a solvent valve, an extract valve, a feed valve, a raffinate valve and optionally an internal reflux valve.